P89LPC901FD datasheet(31/55 Pages) PHILIPS

31 / 55 page

Philips Semiconductors

P89LPC901/902/903

8-bit microcontrollers with two-clock 80C51 core

Product data

Rev. 04 — 21 November 2003

31 of 55

9397 750 12293

8.12.2

Quasi-bidirectional output conﬁguration

Quasi-bidirectional output type can be used as both an input and output without the

need to reconﬁgure the port. This is possible because when the port outputs a logic

HIGH, it is weakly driven, allowing an external device to pull the pin LOW. When the

pin is driven LOW, it is driven strongly and able to sink a fairly large current. These

features are somewhat similar to an open-drain output except that there are three

pull-up transistors in the quasi-bidirectional output that serve different purposes.

The P89LPC901/902/903 is a 3 V device, however, the pins are 5 V-tolerant (except

for XTAL1 and XTAL2). In quasi-bidirectional mode, if a user applies 5 V on the pin,

there will be a current ﬂowing from the pin to VDD, causing extra power consumption.

Therefore, applying 5 V in quasi-bidirectional mode is discouraged.

A quasi-bidirectional port pin has a Schmitt-triggered input that also has a glitch

suppression circuit.

8.12.3

Open-drain output conﬁguration

The open-drain output conﬁguration turns off all pull-ups and only drives the

pull-down transistor of the port driver when the port latch contains a logic ‘0’. To be

used as a logic output, a port conﬁgured in this manner must have an external

pull-up, typically a resistor tied to VDD.

An open-drain port pin has a Schmitt-triggered input that also has a glitch

suppression circuit.

8.12.4

Input-only conﬁguration

The input-only port conﬁguration has no output drivers. It is a Schmitt-triggered input

that also has a glitch suppression circuit.

8.12.5

Push-pull output conﬁguration

The push-pull output conﬁguration has the same pull-down structure as both the

open-drain and the quasi-bidirectional output modes, but provides a continuous

strong pull-up when the port latch contains a logic ‘1’. The push-pull mode may be

used when more source current is needed from a port output. A push-pull port pin

has a Schmitt-triggered input that also has a glitch suppression circuit.

8.12.6

Port 0 analog functions

The P89LPC901/902/903 incorporates an Analog Comparator. In order to give the

best analog function performance and to minimize power consumption, pins that are

being used for analog functions must have the digital outputs and digital inputs

disabled.

Digital outputs are disabled by putting the port output into the Input-Only (high

impedance) mode as described in Section 8.12.4 “Input-only conﬁguration”.

Digital inputs on Port 0 may be disabled through the use of the PT0AD register. On

any reset, the PT0AD bits default to ‘0’s to enable digital functions.

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